PROJECT ABSTRACT The proposed study will help substantially advance understanding of the immunology, biology, and detection of syphilis, through studying newly identified cases of in Lima, Peru, where syphilis is hyper-endemic. Syphilis remains a serious disease with significant adverse clinical outcomes including neurological, ophthalmic, and cardiovascular disease. Furthermore, despite over 100 years of research in the biology of Treponema pallidum, the agent of syphilis, little has been done recently with modern biological methods. Our study builds on the research infrastructure created through a previously-funded NIH capacity-building grant, the ?PICASSO Study? (NIH/NIAID 5R01AI09972), to more deeply investigate the human host immune response to T. pallidum, and fill critical gaps in the understanding of syphilis. There are three specific aims to our proposal. Aim 1: Clinical epidemiology ? Hypothesizing that those with de novo versus repeat syphilis infection will demonstrate different immunological profiles, we will conduct a prospective study of syphilis cases, comparing those with and without a history of prior syphilis infection. We will a) recruit, treat and follow 100 individuals with incident syphilis without prior infection (recently TP seronegative) and 100 individuals with repeat syphilis infection (recently TP seropositive); b) Compare markers of immunobiologic response over time in those 2 cohorts, including RPR and TPPA titers, prototypical inflammatory serum cytokines, and immunologic epitope serum TP antibody assays accounting for HIV- infection status, CD4 T-cell count and HIV viral load. We will also compare cytokine profiles and novel TP antibody expression at the time of serofast status versus new infection among those with similar RPR titers. Aim 2: Biological ? Hypothesizing that the clinical manifestations and immunologic responses (cytokine profiles and antibody responses to TP surface proteins and lipoproteins) will differ between individuals with repeat infection versus de novo incident syphilis infection, active versus treated infection, HIV-associated immunosuppression and TP strain type, we will investigate whether a relationship exists during early syphilis between differential gene expression in TP, development of the immune response to treponemal antigens, host cytokine profiles of disease pathogenesis and immune correlates of infection. Aim 3: Rapid test evaluation ? Using our current biobank (n > 3000 serological and clinical specimens from primary and secondary syphilis- diagnosed patients) and new specimens from study participants from Aim 1, we will evaluate new rapid dual HIV/syphilis rapid tests including combination RPR/TP tests, treponemal self-test kits, and a 10-minute oral mucosal fluid treponemal rapid test. This innovative program will help accelerate syphilis research and potentially clinical practice worldwide, illuminating new insights into syphilis immunology and enhancing opportunities for early detection and clinical management, while informing vaccine development.